Blizzard in Alaska, January 26 to 28, 2020

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKST Sat Jan 25 2020

.ANALYSIS AND UPPER LEVELS...
In the upper levels, a blocking ridge extends from the eastern
Bering Sea northwestward to eastern Russia. High amplitude troughs
are positioned either side of the ridge, one over mainland Alaska
and the Gulf of Alaska and another over the western Aleutians and
northwest Bering Sea. Arctic air is firmly entrenched over
mainland Alaska. Deep northerly flow between low pressure in the
Gulf and high pressure in the Bering Sea is driving the arctic air
southward across Southwest Alaska into the eastern Bering as well
as across the Alaska Peninsula into the western Gulf and down
into the Pacific. This is leading to gusty northerly winds across
southwest Alaska and nearby coastal waters, along with low wind
chills over land areas and heavy freezing spray over marine areas.
As is typical the strongest winds are being observed out of bays
and passes along the south side of the Alaska Peninsula.

A multi-centered low is spinning over the central Gulf, with a
nearly stationary front aligned north to south from the eastern
Gulf down into the east Pacific. Southerly flow aloft on the east
side of the trough is advecting moisture northward across the
eastern Gulf, with some snow showers in the vicinity of Cordova
and on up into the southeastern Copper River Basin. Strong gap
winds, largely supported by tight pressure gradients, are being
observed along the Gulf coast and Prince William Sound as well as
through mountain passes over interior Southcentral.

To the west of the ridge, an old surface front is strung out from
the central Aleutians to the northwest Bering Sea, with a fairly
narrow band of snow right along it. A much larger swathe of gale
force winds can be found north and east if this boundary. The
parent low is nearly stationary near the Kamchatka Peninsula while
a newly formed compact (but deep) low is tracking north-northwest
toward the western Aleutians.

&&

.MODEL DISCUSSION...
Models continue to struggle with the position of multiple weak
lows over the Gulf of Alaska. They are in better (but not great)
agreement on the track of a low tracking up the front across the
eastern Gulf this afternoon through tonight. This will finally
bring some level of confidence to the precipitation forecast for
the Cordova-Valdez-Copper River Basin areas tonight. Models then
break down after this with position and track of numerous lows
crossing the Gulf through Monday night. None of these lows looks
particularly strong. Despite the large model spread, they at least
all show the same general trend, with lows progressing farther
north with time and more precipitation pushing onshore into
Southcentral. Will strive to depict these trends in the latest
forecast.

Model guidance is in much better agreement out west with lows
passing south of the Aleutians and with arctic air spilling south
and west across the Bering Sea over the next few days. There are
some challenges on the local scale with strength of winds as well
as temperatures and wind chills over Southwest Alaska, bit
overall forecast confidence for Southwest Alaska and the Bering
Sea/Aleutians is high.

&&

.AVIATION...
PANC...Expect winds to begin to diminish beginning some time this
evening and continuing overnight as the lows over the Gulf
approach Southcentral and push the core of northerly winds off to
the west. It is looking more and more likely for a period of light
snow during the day Sunday as the upper low center over the Gulf
lifts northward over Southcentral, pulling some of the low level
moisture north and west toward Anchorage. Forcing is weak and any
snow that does fall will be quite light, which makes for a
difficult ceiling/visibility forecast. MVFR seems like worst case
scenario, so have gone on the high end of this category for now.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The main challenges for the current forecast are winds, cold
temperatures, and increasing chances for snow.

Gusty winds continue through channeled terrain and across the Gulf
tonight. Temperatures are forecast to be slightly colder than the
previous night as the upper level low over the Gulf becomes
negatively tilted. This negative tilt of the trough aloft helps
to pull cold air around the core of the system which generally
cools the column of air. This supports colder surface temperatures
through Monday.

For tonight, low pressure over the Gulf keeps ongoing narrow
bands of snow across the central and northern Gulf waters. Snow
spreads inland across Cordova and eastern areas of the Copper
River Valley during the overnight hours as a new surface low
develops and races north from the eastern Gulf. Models are in
better agreement with showing the western extent of frontal
boundary reaching Valdez after midnight. With cold air drainage
expected to continue, a blizzard warning was issued for Thompson
pass as white out conditions are expected during the frontal
passage. Valdez will experience blowing snow also with reduced
visibilities to one mile at times. The surface low driving the
front will quickly dissipate as it slams into the cold air mass
mid morning.

The most challenging question is how far north and west snow will
spread as the core of the upper low positions over the northern
Cook inlet early Sunday afternoon. Low to mid level moisture
streams across Prince William Sound to the Matanuska Valley.
Models were not showing any precipitation making it across the
Chugach Range yesterday, but are now starting to hint at snow
spreading toward Anchorage and the Matanuska Valley. Models
typically struggle on snow prediction when weak surface dynamics
are involved. Have trended the forecast to mention chances for
snow farther west on Sunday as the upper low moves significant
short waves northwestward across the Chugach and Talkeetna Ranges.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The forecast for significant cold air spilling into Southwest
Alaska and the Kuskokwim Delta remains on track. The upper level
high pressure over the eastern Bering and the low in the Gulf of
Alaska are responsible for a tightening pressure gradient and the
resultant northerly winds that will bring dangerous wind chills to
the region on Sunday and Monday. Winds coming over the
southeastern Bering Sea waters will initiate snow showers across
much of the north side of the AKPEN Sunday into Monday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A north Pacific low will continue to generate windy conditions
across the western half of the Aleutian chain. A mix of rain and
snow showers will continue to impact the area as well through
Tuesday.

&&

.MARINE (Days 3 through 5: Tuesday through Thursday)...
Gulf of Alaska: Northerly outflow winds will continue through
bays and passes with areas of gales and heavy freezing spray
across Kamishak Bay and the Barren Islands early Tuesday. On
Tuesday, these winds will relax as a weak low and associated front
pushes inland along the Southcentral Coast. There is increasing
confidence that beginning late Tuesday a ~950mb low will develop
and lift north into the southern Gulf. This system will produce
widespread precipitation with easterly gales (perhaps some storm-
force winds as well) across the Gulf ahead of the low, reaching
the Southcentral Coast by Wednesday. Westerly gales will wrap
around the back side of the low. Waves in excess of 20 feet are
expected with this system.

Bering Sea and Aleutians: Gusty northerly winds will continue
across the eastern Bering through Tuesday with areas of gales and
heavy freezing spray along the Pacific side of the eastern
Aleutians and AKPEN. Gusts will increase along the AKPEN again
Wednesday as a low in the western Gulf strengthens.

&&

.LONG TERM FORECAST (Days 4 through 7)...
There is increasing confidence that a developing storm-force low
will push toward coastal Southcentral Alaska Wednesday. This will
usher in copious amounts of moisture in the form of snow over
much of the coast along with a southeasterly flow and warmer
temperatures. This system will weaken rapidly by late Wednesday as
it pushes onshore and runs into the arctic air entrenched over
interior portions of the Southern Mainland. Given the depth of the
colder air in place, there is still a fair bit of uncertainty
regarding how much of the precipitation makes it over interior
locations. There also remains the possibility that the
southeasterly flow initiates some downsloping over the Kenai and
Chugach and limits the potential for snow in the lee of these
mountains. Snow chances then increase for late Wednesday into
Thursday for the Kuskokwim Valley as the remaining energy from
this feature continues to trek inland and overruns the cold air in
place.

By the end of the week, confidence in the guidance wanes as
solutions diverge. Overall, the pattern remains an active and
unsettled one over Southcentral. The GFS keeps the upper-level
trough axis over the western Gulf, allowing a second deep low to
track north into the Gulf and toward the Southcentral Coast by
late Thursday into Friday. The EC, on the other hand, shifts the
trough axis to the east, shifting the brunt of this second surface
low toward the Alaska Panhandle.

Farther west, arctic air will remain in place over much of
Southwest Alaska, although there is the potential for a brief
period of moderation in the cold temperatures on Thursday as
clouds and light snow move across the region. For the Bering and
Aleutians, an area of low pressure will slide across the Bering
from west to east beginning late Thursday, reaching the AKPEN by
early Saturday. Models are then keying in on a deepening low
southwest of Shemya that looks to swing a front across the western
Bering for Saturday.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 131 (T-Pass).
 Wind Chill Advisory 155 161.
MARINE...Gale Warning 121 127 128 130 131 138 139 150 175>178 185.
 Heavy Freezing Spray 121 127 129>131 137>139 141 150 155 165 179
                 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LB
MARINE/LONG TERM...TM